This invention relates to a low-temperature heat shrinkage multilayered film incorporating a gas barrier layer of a vinylidene chloride type resin. More particularly this invention relates to a novel heat shrinkage multilayered film which exhibits low-temperature heat shrinkage properties (shrinkage and shrink tension), low-temperature toughness, seal strength (seal resistance to oil, seal resistance to heat and drop-bag seal strength at low temperature), gas barrier property, transparency after heat shrinkage, resistance to layer delamination, air-tightness in the clipped state and specular gloss all at high levels.
Heat shrinkage films available for tightly wrapping commodities have long been known. Needs have arisen which expect these films to possess those properties underivable from the resins making up the films. The needs could be met by depositing on these films a suitable combination of layers formed of other resins capable of manifesting the desired properties thereby producing multilayered films which combine the properties possessed severally by the newly added resin layers and those possessed by the original films. Many researches are now under way toward developing multilayered films which meet the needs.
The film properties expected to meet the needs of the market are greatly diversified and increasingly exacted. In contrast, the properties collectively derivable from the component resin layers in multilayered films have their own limits. Besides, the selection of resins for the formation of multilayered films has various restrictions. The unfilled gap between the needs of the market and the properties of the existing films continues to widen. In the circumstance, the appearance of multilayered films capable of fulfilling the exacting needs of the market is strongly in need.
In the field of films used for tightly wrapping fatty foodstuffs such as raw meat, processed meat and cheese, for example, low-temperature shrinkage properties (shrinkage and shrink tension) and gas barrier property currently form targets for special requirements. This is because these fatty foodstuffs in most cases are irregular in shape and have projecting corners, bones and inner packaging materials and the films, when desired to wrap them all up with ample tightness, are required to produce high shirnkage in conjunction with high shrink tension. When the manifestation of these shrinkage properties deviates toward a high-temperature range and has firm dependency upon temperature conditions, the foodstuffs being wrapped are impaired in color and quality or the films sustain wrinkles and sagging because of the phenomenon of uneven shrinkage resulting from uneven temperature distribution. These defects persist on the surface of wrapped products. The low-temperature heat shrinkage films, therefore, are required to remain stably at their normal storage temperatures and, at the same time, manifest their shrinkage properties in a low-temperature range and have no conspicuous dependency on temperature conditions. The gas barrier property which is additionally expected of the films forms a requirement indispensable to prolonged preservation of wrapped commodities. The shrink wrapping of fatty foodstuffs cannot be considered without this particular property.
If a certain film which fulfils the two sets of properties described above lacks transparency and specular gloss or loses transparency (hazing) after heat shrinkage, the wrapped products are impaired in appearance (product value). In the case of a film which is deficient in seal strength, resistance to layer delamination, airtightness in clipped state or toughness needed for low-temperature handling, it fails to discharge the role as a wrapping material for preserving contained foodstuffs for a long time. In the market, therefore, collective fulfilment of all these properties forms a major target for quality requirements. On the part of producers engaging in the supply of multilayered films, design of film compositions capable of satisfying all these properties is extremely difficult from the technical point of view. Thus, the producers are inevitably content with an awkward situation wherein they are compelled to give up some of these essential properties in the manufacture of their films.
Films of vinylidene chloride type resins have found popular acceptance for their superior gas barrier property. They are not faultless, for they are deficient in low-temperature shrinkage, low-temperature toughness, heat seal property, etc. An attempt may be made to relieve this drawback by laminating layers of other resins on such films and enriching their original gas barrier property with the properties of such other resins. To impart the low-temperature shrinkage (low-temperature drawing property) to the films, adoption of the manufacturing process of coextrusion and continued drawing is an inevitable requirement. The adoption of this process ushers in a separate problem that the resins of the newly added layers are required to possess adaptability to the coextrusion with vinylidene chloride type resins used originally and to low-temperature drawing. Particularly, vinylidene chloride type resins lack thermal stability and permit melt extrusion in a narrow low-temperature range. They further involve a problem of poor adhesion with layers of other resins, a problem of heavy loss of drawability due to advance of crystallization, a problem of large evolution of heat during the drawing, and so on. Thus, the selection of other resins capable of imparting desired low-temperature shrinkage properties to the multilayered films of vinylidene chloride type resins becomes extremely difficult because of many restrictions to be entailed. This explains why producers are forced to yield, unwillingly, to the awkward situation that they inevitably give up some of the essential properties in the manufacture of their films.